Code Generation for Control Flow Mooly Sagiv html://msagiv/courses/wcc03.html Chapter 6.4.

Code Generation forControl FlowMooly Sagiv

html://www.math.tau.ac.il/~msagiv/courses/wcc03.html

Chapter 6.4

Outline• Local flow of control

– Conditionals– Switch– Loops

• Routine Invocation

• Non-local gotos

• Runtime errors

• Handling Exceptions

• Summary

Machine Code AssumptionsInstruction Meaning

GOTO Label Jump to Label

GOTO label register Indirect jump

IF condition register then GOTO Label Conditional

Jump

IF not condition register then GOTO Label

Boolean Expressions

• In principle behave like arithmetic expressions

• But are treated specially– Different machine instructions– Shortcut computations

Code for a < b yielding a condition value

Conversion condition value into Boolean

Conversion from Boolean in condition value

Jump to l on condition value

if (a < b) goto l

Shortcut computations

• Languages such as C define shortcut computation rules for Boolean

• Incorrect translation of e1 && e2

Code to compute e1 in loc1 Code to compute e2 in loc2 Code for && operator on loc1 and loc2

Code for Booleans(Location Computation)

• Top-Down tree traversal• Generate code sequences instructions • Jump to a designated ‘true’ label when the

Boolean expression evaluates to 1• Jump to a designated ‘false’ label when the

Boolean expression evaluates to 0• The true and the false labels are passed as

parameters

Exampleif ((a==0) && (b > 5))

x = ((7 * a) + b) if

&& :=

x +

* b

7 a

==

a 0

>

b 5

if

&& :=

x +

* b

7 a

==

a 0

>

b 5

No label Lf

Lt Lf

Load_Local -8(FP), R0

Cmp_Constant R0, 0

IF NOT EQ THEN GOTO Lf

Lt Lf

Load_Local -12(FP), R0

Cmp_Constant R0, 5

IF GT THEN GOTO Lt

GOTO Lf

Lt:

Code for :=

Lf:

Location Computation for Booleans

Code generation for IF

if

Boolean expression true sequence false sequence

Allocate two new labels Lf, Lend

Generate code for Boolean(left, 0, Lf)

Code for Boolean with jumps to Lf

Code for

true sequence

Code for

false sequence

GOTO Lend

Lf:

Lend:

Code generation for IF (no-else)

if

Boolean expression true sequence

Allocate new label Lend

Generate code for Boolean(left, 0, Lend)

Code for Boolean with jumps to Lend

Code for

true sequence

Lend:

Coercions into value computations

:=

x a > b

Generate new label Lf

Load_Constant R0, 0;Generate code for Boolean(right, 0, Lf)

Load_Local -8(FP), R1;CMP R1, -12(FP) ;

IF <= GOTO Lf;

Load_Constant R0, 1Lf: Store_Local R0, -20(FP)

Effects on performance

• Number of executed instructions

• Unconditional vs. conditional branches

• Instruction cache

• Branch prediction

• Target look-ahead

Code for case statements

• Three possibilities– Sequence of IFs

• O(n) comparisons

– Jump table • O(1) comparisons

– Balanced binary tree • O(log n) comparisons

• Performance depends on n• Need to handle runtime errors

Simple Translation

Jump Table

• Generate a table of Lhigh-Llow+1 entries– Filled at ?time

• Each entry contains the start location of the corresponding case or a special label

• Generated codetmp_case_value:= case expression;

if tmp_case_value <Llow GOTO label_else; if tmp_case_value>Lhigh GOTO label_else; GOTO table[tmp_case_value –Llow];

Balanced trees

• The jump table may be inefficient– Space consumption– Cache performance

• Organize the case labels in a balanced tree– Left subtrees smaller labels– Right subtrees larger labels

• Code generated for node_k

label_k: IF tmp_case_value < lk THEN GOTO label of left branch ; IF tmp_case_value >lk THEN GOTO label of right branch; code for statement sequence; GOTO label_next;

Repetition Statements (loops)

• Similar to IFs• Preserve language semantics• Performance can be affected by different

instruction orderings• Some work can be shifted to compile-time

– Loop invariant– Strength reduction– Loop unrolling

while statements

Boolean expression

while

statement

Sequence

test_label:

Generate new labels test_label, Lend

Generate code for Boolean(left, 0, Lend)

Code for Boolean with jumps to Lend

Code for

statement sequence

GOTO test_label;

Lend:

while statements(2)

Boolean expression

while

statement

Sequence

GOTO test_label:Ls:

Generate labels test_label, Ls

Generate code for Boolean(left, Ls, 0)

Code for Boolean with jumps to LS

Code for

statement sequencetest_label:

For-Statements

• Special case of while

• Tricky semantics– Number of executions– Effect on induction variables– Overflow

Simple-minded translation

FOR i in lower bound .. upper bound DO statement sequence

END for

i := lower_bound;

tmp_ub := upper_bound;

WHILE I <= tmp_ub DO code for statement sequence

i := i + 1;END WHILE

Correct TranslationFOR i in lower bound .. upper bound DO statement sequence

END for

i := lower_bound;

tmp_ub := upper_bound; IF i >tmp_ub THEN GOTO end_label;

loop_label: code for statement sequence if (i==tmp_ub) GOTO end_label;

i := i + 1;

GOTO loop_label;

end_label:

Tricky question

Loop unrolling

FOR i := 1 to n DO

sum := sum + a[i];

END FOR;

Summary• Handling control flow statements is usually

simple

• Complicated aspects– Routine invocation– Non local gotos– Runtime errors

• Runtime profiling can help

Routine Invocation

• Identify the called routine• Generate calling sequence

– Some instructions are executed by the callee

• Filling the activation record– Actual parameters (caller)– Administrative part – The local variable area– The working stack

Parameter Passing Mechanisms

• By value

• By reference

• By result

• By value-result

• Pass the R-value of the parameter

• Pass the L-value of the parameter

• Pass the L-value of the parameter

• The callee creates a temporary

• Stores the temporary upon return

• Can use registers

• Pass the L-value of the parameter

• The callee creates a temporary

• Store the temporary upon return

Caller Sequence

• Save caller-save registers• Pass actual parameters

– In stack– In register

• Pass lexical pointer• Generate code for the call

– Store return address– Pass flow of control

Callee Sequence

• Allocate the frame

• Store callee-save registers

• Perform the procedure code

• Return function result

• Restore callee-save registers

• Deallocate the frame

• Transfer the control back to the caller

Two activation records on the stack

Non-Local goto in C syntax

Non-local gotos

• Close activation records

• Restore callee-save registers

Main p:

l:

q:

goto l;

code Memory before

p

r

q

Main

Memory After

p

Runtime errors

• The smartest compiler cannot catch all potential errors at compile-time– Missing information– Undecidability

• Compiler need to generate code to identify runtime errors– Non-trivial

Common runtime errors

• Overflow– Integers

– Stack frame

• Limited resources• Division by zero• Null pointer dereferences• Dangling pointer dereferences• Buffer overrun (array out of bound)

Runtime Errors

• C– No support for runtime errors

– Situations with “undefined” ANSI C semantics• Leads to security vulnerabilities

• Pascal– Runtime errors abort execution

• Java– Runtime errors result in raised exceptions

– Can be handled by the programmer code

Detecting a runtime error

• Array bound-checkfoo() { int a[100], i, j; scanf(“%d%d”, &i, &j); while (i < j) { a[i] = i ; i++;}

if (“i” < 0 || “i” >=100)

THROW range_error;

Detecting a runtime error(2)

• Array bound-checkfoo() { int a[100], i, j; scanf(“%d%d”, &i, &j); if ((i >=0) && j <=100) { while (i < j) { a[i] = i ; i++; } else …}

Handling Runtime Errors

• Abort

• Statically assigned error handlers (signal)

• Exceptions

Signal Handlers

• Binds errors to handler routines

• Invoke a specific routine when runtime error occurs

• Report an error– Close open resources and exit– Resume immediately after the error– Resume in some “synchronization” point

Signal Example

Exceptions

• Flexible mechanism for handling runtime errors• Available in modern programming languages• Useful programming paradigm• But are hard to compile

void f() {

{ … g() …

catch(error1) {

… }

}

}

void g() {

… h() …

}

void h() {

… throw error1 …

}

Why are exceptions hard to compile?

• Dynamic addresses– Not always known when at compile time– Non local goto– Register state

• The handler may change in the execution of a routine

• The handler code assumes sequential execution

Handling Exceptions

• At compile time store mappings from exceptions to handlers

• Store a pointer to the table in the activation record

• When an exception is raised scan the stack frames to locate the most recent handler

• Perform a non-goto

• Generate code for handler

• Terminate with a jump to the end of block/routine

• A unique label where the handler code begins

• Generate a table

• Store a pointer to the table at the activation record

Handler Code Block/Routine

exception handler

Code for raising exception

• Extract the pointer to the table from the activation record

• Search for a handler for the exception raised

• If not found pop the stack and repeat

• If found perform a non-local goto

• Usually combine the search and the goto

Summary

• Non local transfer of control can be expensive– Hard to understand = Hard to implement

• But are necessary

• Challenging optimization problems